Aluminothermic production of magnesium and an oxidic slag containing recoverable alumina

ABSTRACT

AN ALUMINOTHERMIC PROCESS FOR THE PRODUCTION OF MAGNESIUM BY THE REDUCTION OF MAGNESIUM OXIDE FROM A MIXTURE OF MAGNESIUM OXIDE AND CALCIUM OXIDE, BY MEANS OF A METALLIC REDUCING AGENT COMPRISING AT LEAST 85 PERCENT ALUMINUM, IN THE PRESENCE OF A MOLTEN CALCIUMALUMINATE SLAG BATH AT A TEMPERATURE OF ABOUT 13001700*C. MAGNESIUM EVOLVES FROM THE MOLTEN SLAG AS A VAPOR. THE PROCESS MAY BE OPERATED CONTINUOUSLY AND AT ATMOSPHERIC PRESSURE. THE MOLTEN SLAG RESIDUE IS TAPPED PERIODICALLY, WHEN THE MGO LEVEL IS BELOW 5 PERCENT, AND HAS A COMPOSITION OF 35-65 PERCENT AL2O3, 35-55 PERCENT CAO AND 0-10 PERCENT SIO2 (BY WEIGHT). THE DISCLOSURE DEMONSTRATES HOW THE RATIO OF MAGNESIUM OXIDE TO CALCIUM OXIDE IN THE CHARGE AND THE QUANTITY AND COMPOSITION OF THE METALLIC REDUCING AGENT ARE INTERRELATED IN SUCH A WAY THAT (1) VIRTUALLY ALL OF BOTH THE REDUCING AGENT AND MAGNESIUM OXIDE ARE CONSUMED IN THE REDUCING REACTION, AND (2) THE CALCIUM-ALUMINATE SLAG PRODUCED IS OF SUCH A COMPOSITION THAT PURE ALUMINA CAN BE RECOVERED FROM IT WITH A VERY HIGH YIELD.

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United States Patent 3,782,922 ALUMINOTHERMIC PRODUCTION OF MAGNE- SIUMAND AN OXIDIC SLAG CONTAINING RECOVERABLE ALUMINA Julian Miles Avery, 47Old Orchard Road, Chestnut Hill, Mass. 02167 Continuation-impart ofapplications Ser. No. 796,214, Feb. 3, 1969, now Patent No. 3,658,509,dated Apr. 25, 1972, Ser. No. 26,118 Apr. 6, 1970, Ser. No. 143,886 andSer. No. 144,321, both May 17, 1971, all of which in turn arecontinuation-imparts of application Ser. No. 648,856, June 26, 1967, nowPatent No. 3,579,326, dated May 18, 1971. This application Apr. 18,1972, Ser. No. 245,142

Int. Cl. C22b 45/00 US. CI. 75-67 14 Claims ABSTRACT OF THE DISCLOSUREAn aluminotherrnic process for the production of magnesium by thereduction of magnesium oxide from a mixture of magnesium oxide andcalcium oxide, by means of a metallic reducing agent comprising at least85 percent aluminum, in the presence of a molten calciumaluminate slagbath at a temperature of about 1300- 1700 C. Magnesium evolves from themolten slag as a vapor. The process may be operated continuously and atatmospheric pressure. The molten slag residue is tapped periodically,when the MgO level is below percent, and has a composition of 35-65percent A1 0 35-55 percent CaO and 0-10 percent Si0 (by weight). Thedisclosure demonstrates how the ratio of magnesium oxide to calciumoxide in the charge and the quantity and composition of the metallicreducing agent are interrelated in such a way that (1) virtually all ofboth the reducing agent and magnesium oxide are consumed in the reducingreaction, and (2) the calcium-aluminate slag produced is of such acomposition that pure alumina can be recovered from it with a very highyield.

CROSS REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of 1) Ser. No. 796,214, filed Feb. 3, 1969,patented Apr. 25, 1972, No. 3,658,509, directed to the use of inert gasin the vapor space above a molten oxidic slag bath for themetallothermic production of magnesium; (2) Ser. No. 26,118, filed Apr.6, 1970, directed to the use of a substantially static atmosphere ofinert gas in the vapor space above a molten slag for the metallothermicproduction of magnesium; (3) Ser. No. 143,886, filed May 17, 1971,directed to the metallothermic production of magnesium induced by astream of inert gas in a system containing a molten oxidic slag; and (4)Ser. No. 144,321, filed May 17, 1971, directed to the metallothermicproduction of magnesium from a molten oxidic slag wherein hydrogen isused in the vapor space above the slag, all of the above applicationswere in turn continuation-imparts of Ser. No. 648,856, filed June 26,1967, now Pat. No. 3,579,326, issued May 18, 1971, directed to thereduction of magnesium oxide to magnesium with an aluminumsilicon alloyin the presence of an acidic molten oxide slag.

SUMMARY OF THE INVENTION An aluminothermic process for the production ofmetallic magnesium from oxidic ores using a reducing agent consisting ofmetallic aluminum or one or more aluminum alloys containing at leastpercent aluminum by weight, in the presence of a by-product,calcium-alurninate slag from which aluminum oxide can be readilyrecovered. General features of the process are:

(a) The reduction reaction is carried out in an internally heatedelectric furnace, at a temperature between about 1300 C. and 1700 0,preferably about 1500 C.

(b) Magnesium vaporized by the reaction is recovered in a condenser andcollected in a crucible, preferably as molten magnesium.

(c) The entire system of furnace, condenser and crucible is preferablymaintained at about atmospheric pressure. The magnesium may be evolvedfrom the molten slag at a partial pressure of about atmospheric pressureand condensed by suitable procedures, or, alternatively, at a lowerpartial pressure in a system containing inert gas. Either alternativepermits continuous or nearly continuous operation, since at atmosphericpressure molten slag and magnesium can readily be removed from thefurnace and the crucible.

(d) The oxidic and metallic raw materials are so proportioned to producea slag of such composition that the alumina contained in the molten.slag will crystallize, as the slag cools, in the form predominately ofsoluble calcium aluminates, i.e. CaO-Al 0 (64.5% A1 0 or 12CaO-7Al O(51.5% A1 03), or both.

The formation of such a slag requires that the ratio in the oxide chargeof MgO to CaO be about 1.1 to 2.3 by weight. If dolomite is used as thesource of CaO for the process, the corresponding weight ratio of dolimeto magnesia is about 1.2 to 3.8. Alternatively, magnesia (MgO) and lime(CaO) may be used.

It is within the scope of the present invention to produce such slagseither by using low grade magnesia, e.g., siliceous magnesite, by usingmetallic silicon to supplement aluminum as the reducing agent, or byusing an aluminum alloy containing some silicon.

However, the production of a silica-containing slag should not be apreferred form of the invention (because alumina recovery is higherwithout it), but it is an efficient modification to utilize cheap, lowgrade magnesite, or scrap aluminum containing silicon, or to employ whenalumina recovery is not desired.

While extraction of A1 0 from calcium-aluminate slags is not essentialto the practice of the present invention, it is an important adjunct ofthe process. U.S.B.M. studies have shown that to achieve the purpose itis not necessary to produce a disintegrating slag containing silica. Infact, slags composed of the pure oxidic compounds Ca0-Al O and 12CaO-7AlO and presumably mixtures thereof, can be leached with nearly percentefficiency with Na CO solution (no free NaOH) at 70 C. in about 1 hour,with practically no residue except the CaCO, formed by the reaction ofNa CO to dissolve A1 0 as sodium aluminate.

Carbonation to precipitate Al( OH) from the solution regenerates the NaC0 and the solution may be recycled with whatever make-up Na CO may berequired due to losses.

Comparison of this extraction process with the usual commercialprocesses (Bayer and Pedersen) leads tothe conclusion that it is simplerand cheaper to operate and requires less capital investment than either.A further advantage of the proposed low silica or silica-free slag ofthe invention is that it makes possible the production of cell-grade A1Without a separate desilication step, which is sometimes necessary whensilicous bauxites are processed. Thus the A1 0 content is more valuableper unit than the A1 0 content of high grade bauxite used in thecommercial Bayer process, and considerably more valuable than the A1 0content of low grade bauxites used for the Pedersen process. Moreover,it seems quite possible that the leaching operation can be carried outin an existing alumina plant, even though some of the equipment will besuperfluous.

An important feature of the present process is that it can use as areducing agent cheap aluminum scrapeither captive scrap from aluminumproduction plants or scrap purchased from aluminum fabricating plants.At once the question arises whether the use of such scrap mightadversely affect the technology of the process or the quality of themagnesium metal produced, because of the alloying agents present in themany types of alloyed aluminum used for fabrication. A survey of thecompositions of the usual alloys shows that the principal alloyingmetals used are magnesium, silicon, copper, manganese, chromium andzinc, in amounts generally less than 5 percent but up to 12 percent inat least one alloy.

Finally, there is the question of aluminum itself as an impurity in themagnesium product, since aluminum has a vapor pressure of about mm. Hgat 1500 C. This means that magnesium produced by the present processwill inevitably contain aluminumhow much depends upon the operatingtemperature. At 1400 C., for example, it would contain about 0.5percent, at 1500 C. about 1.2 percent and at 1600 0., about 2 percent ofaluminum. However, this is not a serious problem and may in fact bebeneficial, because: (a) the principal use for magnesium today is toproduce aluminum alloys for fabrication; and (b) a major portion of themagnesium used for fabricated magnesium products contains a substantialproportion of aluminum-generally from 3 to 9 percent. Thus the presenceof a small amount of aluminum in the magnesium produced by the presentprocess is not detrimental, especially if the magnesium operation isassociated with the production of aluminum, which is likely to be thecase because of the advantage of recovering A1 0 from the slag produced,and the possibility of using captive scrap as the reducing agent.

From this point of view the process of the invention might becharacterized in its preferred form by its overall results: theconversion of scrap aluminum into virgin magnesium coupled with theconversion of magnesium oxide into cell-feed alumina.

BACKGROUND OF THE INVENTION There are two known commercial processes forthe metallothermic production of magnesium; both are batch operationscarried out under very high vacuum. Neither has been operated on a largescale comparable to the generally practiced production of magnesium byelectrolysis of molten magnesium chloride.

The object of the present invention is to provide a metallothermicprocess which can compete successfully with the electrolytic process oneither a large or a small scale. It is especially adapted for use by acompany which manufactures and fabricates aluminum, because of theavailability of captive scrap, facilities for recovering alumina fromthe slag, and a need for a high grade alumina for aluminum cell feed.But it can also be operated independently, particularly if based upon anadequate supply of scrap aluminum at low cost.

In one of the present commercial metallothermic processes*, known as thePidgeon Process, ferrosilicon The various thermal reduction processes,including those detailed herein, are described with bibliography topertinent patents and publications in Principles of MagnesiumTechnology, E. F. Emley, Pergamon Press (London, 1966), pp. 46-64; and acomparison is given between the electrolytic and metallothermicprocesses at pp. 65-67.

(about percent Si) and dolomite lime are charged into a battery ofexternally fired horizontal tubular retorts, and the magnesium iscollected as a crown by condensation in an extension of the retort.Because the retorts cannot withstand very high temperatures, thereaction is necessarily a solid state reaction, and at a relatively slowrate. This process has high capital investment and operating cost, andis used only where the exceptionally high quality of the magnesiumproduced can command a premium price.

The other present commercial metallothermic process, known as theMagnetherm Process, also uses dolomitic lime as the source of MgO andferrosilicon (about 78 percent) as the reducing agent. The reduction iscarried out at about 1500 C. in the presence of a moltencalciumaluminum-silicate slag, in an internally heated electric furnace.The ratio of slag to magnesium is very highabout 6 to 1, which creates adisposal problem unless it can be sold or used to produce cement, forwhich it has little value. The A1 0 required to produce a slag of thedesired composition, must be added to the furnace as either alumina orbauxite. The process is a batch operation, due to the necessity ofcutting off power and breaking vacuum in order to tap slag halfwaythrough the batch, and to tap slag and spent ferrosilicon and to removethe magnesium crucible at the end of the batch. Under very high vacuum,air leaks into the system, and magnesium is lost, not only by oxidationor nitridation, but also due to problems associated with the condenserand crucible, which must be removed, cleaned and replaced at the end ofeach batch.

The present invention provides a process which can be operatedcontinuously at atmospheric pressure with high overall recovery ofmagnesium, which produces a co-product calcium-aluminate valuable as asource of alumina-both at low capital and production costs.

A so-called modified aluminothermic process, called the MC process (seeEmley, supra, p. 50; British Pat. No. 922,300; Light Metals (February1964), p. 44), has also been posed and briefly described. The processand its modifications, as described, have several disadvantages. Theslag formed in one such process (see Brit. Pat. 922,- 300), because itcontains 15 percent MgO, cannot practically be used to recover A1 0 ascan the present slag.

Certain experiments relating to the reduction of magnesium oxide oreswith aluminum are reported in Trans. Can. Inst. Min. Met, 1962, 65, pp.221-224. The purpose of the experiments was to determine the vaporpressure of the magnesium product obtained by the reduction of magnesia,dolomite and olivine with aluminum. These studies conclude that thereaction product of the reduction of magnesia is magnesium aluminate,and that the reaction product of reduced dolomite is either (below 1200C.) or 3CaO-Al O (above 1200 C.). Contrary to these results, I havefound that it is possible to obtain at a temperature of about l300l700C. a calcium-aluminate slag predominately containing 12CaO 7Al O CaO A10 or mixtures thereof. Alumina can be readily recovered from both ofthese oxide products, or mixtures thereof,

Whereas the product suggested in the reference,

3CaO-Al O is insoluble and does not permit the recovery of aluminatherefrom.

The present invention offers numerous advantages over theabove-mentioned metallothermic processes. First, virtually all of thealuminum reducing agent can be consumed in the primary reaction, therebyavoiding the necessity of recycling or disposal of considerablequantities of spent metal, such as iron alloys when ferrosilicon orferro-aluminum is used. Second, the magnesium oxide charge, preferably amixture of magnesia and calt 5 cined dolomite, can contain a desirablyhigh ratio of MgO to CaO. Together, these advantages mean that thefurnace capacity is much higher than with previous processes. Third, thealuminum reducing agent is far more reactive, and fourth, the moltenslag can be fluid at a relatively low temperature; which advantagespermit operation at relatively low temperature in a molten slagenvironment with the evolution of magnesium vapor at about atmospheric(reaction-equilibrium) pressuremaking feasible a continuous orsemi-continuous process. A fifth and very important advantage is therecoverability of alumina from the resulting slag, which in turn reducesthe net ratio of slag to magnesium substantially, to between about 3 tol and 2 to 1, or even lower.

In another art, in commercial use on a small scale, is the PedersenProcess, for the production of alumina for aluminum cell feed from lowgrade bauxite (ferruginous or siliceous or both) by smeltingbauxitetogether with lime and carbonaceous reducing agent in asubmerged-arc type electric furnace to produce a low grade iron productand a calcium-aluminate slag from which Al O can be recovered.Unhappily, the A1 recovered contains SiO -sufficiently troublesome in analuminum cell feed, in some cases, to require a separate desilicationtreatment. See H. E. Blake, Jr., Bur. Mines Rept. Inv. 6939 (1967). Incontrast, the slag herein pro duced may be silica-free, and pure aluminamay be recovered without desilication.

BRIEF DESCRIPTION OF THE INVENTION The present invention may becharacterized as an aluminothermic process for the production ofmagnesium metal and a calcium-aluminate slag containing recoverablealumina by the reduction of magnesium oxide with substantially purealuminum in a reaction zone at an elevated temperature wherein: theoxide charge contains MgO and CaO in a weight ratio of between about 1.1and 2.3; the reducing agent comprises at least 85 percent aluminum; andthe slag produced as a by-product of the reaction has a composition ofabout 35-65 percent alumina, 35-55 percent calcium oxide and 0-10percent silica and, when removed from the system, contains less than 5percent magnesia.

This process is capable of substantially continuous op eration at aboutatmospheric pressure. The process also allows substantially fullutilization of the magnesia and reductant charged. The weight ratio ofslag to magnesium may be held to a range of about 2-3z1. Pure aluminumoxide may be extracted from the slag, since when cooled and solidifiedit is composed primarily of the soluble calcium aluminates, 12CaO-Al Oand CaO-A1 O When aluminum oxide is recovered from the slag, theresidual CaCO (to magnesium) ratio will generally range from about 2.8:1to 1.4:1. Moreover, the residual CaCO can be converted to CaO andrecycled-thereby decreasing the net slag (i.e. solid residue) ratioalmost to zero.

BRIEF DESCRIPTION OF THE DRAWINGS The attached figures are presented toillustrate a complex process embodying the present invention inpreferred form.

FIG. 1 is a block diagram of that part of the present process whereinmagnesia and calcined dolomite are reacted in a magnesium furnace withsubstantially pure aluminum to obtain magnesium by vaporization andcondensation and a calcium-aluminate slag by-product.

FIG. 2 is a block diagram of part of the present process, wherein thecalcium-aluminate slag by-product is leached in order to obtain purifiedalumina.

DETAILED DESCRIPTION OF THE INVENTION General definitions Several termsare used herein which need definition. The reducing agent in themagnesium furnace is substantially pure aluminum. This includes not onlypure 3003, 2017, 2024, 5052, 6063 :and 7075). In general,

silicon may constitute up to 10 percent or even slightly more of thereducing agent (eg 12 percent in alloy 13), if a disintegrating slag isdesired. It is preferred that the remaining components, such asmanganese, calcium, chromium or magnesium, constitute less than 5percent of the reducing agent by weight.

The term calcium-aluminate slag refers to that part of the reactionmedia constituting the oxide by-products of the reaction. These oxidesare principally calcium oxide and alumina, but may include silica orother oxides. As herein defined only the calcium oxide, alumina andsilica are significant, and the composition will contain as variouscomplex oxides: 35-55 percent CaO, 35-65 percent A1 0 and 0-10 percentSiO Up to 5 percent or even more of other oxides may be disregarded.However, the slag as tapped must not contain more than 5 percent MgO,and, if alumina is to be recovered in reasonable yield, no more than 2percent MgO and preferably as little as possible. The term moltenapplied to such slag means that at least a major part of the slag is ina liquid state. This generally occurs in the range of composition forthe present invention at 1300-1700 C.

Other terms used herein, such as inert gas and substantially static, aredefined in my co-pending applications Ser. Nos. 796,214 and 26,113.

The calcium aluminate slag by-product A vital element in the presentprocess is the calciumaluminate slag, which together with the meltedreducing agent constitutes the liquid medium in the magnesium furnace.The composition of the slag is important from two aspects. First, it isessentially the reaction medium wherein magnesium oxide is reduced tomagnesium and concurrently in turn the reducing agent aluminum isoxidized to alumina. (Any silicon present in the reducing agent may ormay not be oxidized.) Second, if the composition of the slag is properlycontrolled, it can be a valuable by-product, providing alumina for useas cell-feed or other purposes such as high quality refractories.

In the Al O -CaO-SiO tertiary system, CaO.Al O and 12CaO.7Al O arealmost completely soluble in a leaching solution of sodium carbonate;but 3Ca.Al O is less than 50 percent soluble, Ca0.2Al O is almostcompletely insoluble and 2CaO.Al O .SiO is insoluble. To maximizealuminum oxide recovery, it is desirable to produce a spent oxide withas much as possible of its aluminum oxide content in the form ofcrystalline calcium aluminates. See generally, Bur. Mines Rept. Inv.6939, supra, 4-13 and Recovery of Alumina and Iron from PacificNorthwest Bauxitesby the Pedersen Process, 0. C. Fursman et al., Bur.Mines Rept. Inv. 7079, 1968, 4-15.

Controlled cooling of the molten slag maximizes alumina recovery. Theproper cooling rate is a function of slag composition; generally, a slagwith a substantial silica content must be cooled more slowly than onewith a lower amount or none. Further, it is desirable that the coolingrate be slow enough so that the two phase transformations of dicalciumsilicate (2CaO.SiO alpha to beta at 1420 C. and beta to gamma at 675 C.,be allowed to occur, since the inversion to the gamma form isaccomplished by a 10 percent increase in volume, which can shatter thecooled residue into a fine dust, a phenomenon commonly called dusting.Too rapid cooling prevents crystallization and produces a glassy solidfrom which aluminum oxide extraction is poor.

*Magnesium may be present in the reducing agent but, as should beobvious, it should not count in determining its composition for thepurposes of this invention. It will be distilled into the product.

The silicon dioxide content of the slag must be at least about 5 percentto effect dusting. Dusting greatly reduces the amount of crushingrequired before the aluminum oxide can be extracted. However, as shownin the examples, a very high recovery of aluminum oxide, up to 97.5percent, is possible when the silicon dioxide content is less than 4percent, too low for appreciable dusting. Thus, the desirability ofhaving enough silicon dioxide for dusting must be balanced against thedesirability of the high total recovery of aluminum oxide. (If the slagcontains enough Si to cause dusting, the A1 0 product is likely tocontain more SiO than is permissible in cell feed, and a separatedesilication step may be required.) The silica may be not only derivedfrom silicon, but may be alternatively that silica from the originaloxide charge (e.g., magnesite). On balance, silica is neither necessarynor preferred.

The aluminium oxide content of the slag should be high, to minimize theamount of slag to be processed per unit of A1 0 recovered. The calciumoxide content should be low, and need not exceed that needed to formsoluble calcium aluminates.

The presence of magnesium oxide in the slag, however, severely affectsrecovery of aluminum oxide, because it has been found that a mole of MgOcan tie up four moles of A1 0 in the insoluble complex Thus, the slag istapped periodically when its MgO content has been lowered to less than 5percent. Preferably substantially all of the magnesium oxide is depletedfrom the reaction zone (leaving less than 2 percent) before the slag isremoved.

Weighing all these factors, the range for the tertiary composition ofthe spent oxides, exclusive of minor amounts of other oxides (ifpresent), is as follows: 35- 65 percent aluminum oxide, 35-55 percentcalcium oxide and 0-10 percent silicon dioxide.

Alumina recovery from slag by-product Exhaustive studies of the Pedersenprocess have been carried out by the U.S.B.M. with the objective ofdetermining whether it can be used successfully for the recovery of A1 0at reasonable cost from low quality alumina-bearing ores such as lowgrade bauxite and clay. We are not concerned here with the smeltingoperation for the production of slag, which has no counterpart in thepresent process, but only with the leaching operation.

Results of immediate interest here may be summarized:

For high leachability it is not necessary to produce a distintegratingslag. The oxidic compounds CaO.Al O and 12CaO.7Al O when producedsynthetically in crystal form and properly ground, are completelysoluble, or nearly so, in straight Na CO solutions (no free NaOH) attemperatures of about 70 C. and a leaching time of one or two hours. Allother calcium aluminate compounds are either insoluble or diflicultlysoluble in such a solution.

Within the range of slag compositions given above, the two calciumaluminates shown are the only ones which are formed. In order to obtainthem in crystalline form, the slag must be cooled very slowly, but thiscan easily be done.

If there is a substantial amount of SiO in the slag, usefulness of theA1 0 as aluminum cell feed is restricted. In some cases this problemmust be overcome by a separate desilication step, which increases thecost of A1 0 Avoidance of this problem is a major reason for theproduction of an essentially SiO -free slag as the preferred form of theprocess of the invention.

If MgO is present in the slag, even in small proportions, it has a verydeleterious effect upon the recovery of A1 0 As the slag cools, there isformed a complex oxidic compound, 6CaO-4Al O -MgO-SiO which ispractically insoluble in the leaching operation. Thus one mole of MgO inthe slag not only ties up four moles of A1 0 with loss of recovery; italso ties up six moles of CaO. However, the activity of Al in the systemis so high that a finished slag containing little or no MgO can beproduced. This may require a brief finishing-off period before the slagis tapped.

The substantially pure aluminum reducing agent Substantially purealuminum permits vigorous reduction of magnesium oxide to magnesium andthe evolution of magnesium vapor from the molten slag at about oneatmosphere total pressure. As mentioned above, the aluminum may becomposed of pure metal, high-aluminum alloys or aluminum scrap metal,including alloys. The compositions of the principal aluminum alloys andaluminum scrap materials produced by their fabrication are given belowin Table l.

The aluminum reducing agent may be derived, in part, by reduction ofalumina recovered from the slag byproduct to obtain aluminum to berecycled to the reducing agent feed.

It might be supposed that the presence of metallic impurities inaluminum scrap used as reductant in the present process would presentserious or even insuperable problems in its use for such a purpose. Infact, this is not the case, and most of these impurities providepositive credits either to the cost of production or to the total valueof the magnesium produced.

TABLE I.COMPOSITION OF ALUMINUM ALLOYS Number The low-price scrapmaterials may be used readily in the present process, and this factor isvery important to its economics. The principal impurities in thealuminum alloys and scraps are magnesium, silicon, copper, manganese,chromium and zinc, usually less than 5 percent by Weight. Alloy 13 has12 percent silicon and alloy 220 has 10 percent magnesium, but plainlythese impurities present no problem and can be utilized fully in thepresent process.

Copper is present in proportions up to 4.5 percent in several alloys; itis inert in the process and will form part of the molten metal heel inthe bottom of the furnace. The same holds for chromium which is presentin small amounts in a few alloys.

Manganese is present in amounts ranging from 0.6 to 1.2 percent in a fewalloys, and since its vapor pressure is about 10 mm. Hg at 1500 C., somemanganese vapor will be carried over with the magnesium vapor into theproduct. Since actual operations use a mixture of all kinds of scrap,most of which do not contain manganese, it is improbable that thisimpurity will create serious problems. But if it does, then eithermanganese-bearing scrap must be eliminated, or used separately toproduce magnesium for use as alloys of aluminum containing bothmanganese and magnesium-of which there are several.

The impurity which could cause trouble is zinc, which has a boilingpoint of 907 C., so that any zinc present in the scrap will be carriedover into the magnesium produced. However, there appears to be only onealloy containing zinc (about 5.6 percent).

The oxide charge containing magnesia The oxide charge contains magnesiaand preferably calcmed dolomite. The magnesia is of course reduced by 9the substantially pure aluminum reducing agent. The ratio of MgO to CaOin the charge is about 1.1 to 2.3 by weight. Dolime provides bothconstituents in a 1:1 mole ratio; magnesia may be used to supply MgO,and lime, to supply CaO.

The calcium oxide is carried through to the slag 'byproduct where, uponsolidification, it combines with A1 to form the desired calciumaluminates. Part, or even all, of the required calcium oxides may beadded as lime, but this is not generally preferred, since usuallydolomitic lime is available to provide not only the required calciumoxide but also magnesium oxide.

Low cost magnesia can be obtained in several ways, at suitablelocations: (a) production of MgO from sea water, brines, or bitterns bymethods which are well known; (b) the use of low-grade magnesite notsuitable for refractory purposes and therefore available at low cost.Such ores are generally available as waste from refractory grademagnesia operations, or as low grade ore deposits not presently worked.Such low grade ores usually contain both CaO and SiO; in relativelysmall proportion to MgO.

The presence of CaO in small proportion in such ores is of course notdetrimental, because CaO is required in the process, but the presence ofSiO requires careful consideration. If SiO is present in acalcium-aluminate slag of the type contemplated, when the slagsolidifies it will form the relatively insoluble compound 2CaO-SiO Inproportions from about 5 percent to percent SiO by weight the result isa slag which disintegrates or powders when cooled very slowly to permitphase changes of the crystalline silicate to take place.

Furnace construction An important problem incidental to the presentprocess is the provision of a refractory furnace lining which will holdthe molten slag at 1500 C. or more, by forming a self-perpetuating solidlining between the graphite lining which protects the furnace shell, andthe molten slag, which would otherwise attack the graphite withdisastrous results.

In the Magnetherm process, magnesia is used for this purpose, and itserves very well because it is not appreciably attacked by the high-limecalcium aluminum silicate slag used in that process, whereas the moltencalcium aluminate slag used in the present process may be expected toattack MgO quite readily. However, a study of the slag phase diagram forcalcium aluminate slags shows that if alumina is used as the refractory,any attack upon it by the molten slag will cause an increase in the A1 0content of the film of slag at the surface of the lining, with theresult that eventually an inner shell of slag containing more than about65 percent Al O will form and soldify. This happens because of thefortunate circumstance that the lowest melting point of calciumaluminate slags is 1400 C. at about 54 percent A1 0 which is near themedian of the proposed range of slag composition, and as the A1 0content of the slag increases the melting point increases steadily andsharply to 1550 C. at 60 percent A1 0 and 1650 C. at 70 percent A1 0Modifications dependent upon economic considerations The economics ofthis process are such that if in special circumstances scrap aluminumand other raw materials are cheap enough, and A1 0 is below its normalvalue, the slag can be discarded or sold at a low price. In such case,it might be desirable to use an alloy with silicon content up to 10percent to produce a disintegrating slag.

It is also possible that situations exist where low quality magnesite orthe like is available at such low cost as to alfect the entire economicsof the process. Such magnesites are available at numerous places,especially as waste or discard from the production of refractory gradeMgO. Deposits of magnesite or magnesitic dolomites, un-

i0 suitable as a source of refractory grade magnesia, are also availablefor use herein.

Such magnesites usually contain both SiO and CaO as the undesirableimpurities. For use in the present process the 'CaO is not onlyacceptable but has some small value. The Si0 content, within reasonablelimits, can be used to produce a distintegrating slag from which A1 0can be recovered. As with the case mentioned above, it is a matter ofeconomics whether the low cost of MgO, plus the cost of pulverizing anondisintegrating slag, counterbalances the somewhat lower recovery ofA1 0 and somewhat higher leaching costs, plus the disadvantage of anundesirably high Si0 content of the A1 0 produced.

Other conditions The residual aluminum, in the slag during thefinishing-olt period, will be distilled over into the magnesium.

During the finishing-off operation, neutral gas is introduced into thesystem to replace magnesium vapor and maintain atmospheric pressure, butin a properly designed system this will be automatic and in any casedoes not represent a serious operating problem. When slag or magnesiumare tapped from the furnace or crucible, the pressure of neutral gas canbe raised above atmospheric to force them out of the system without useof a pump. If the use of scrap aluminum results in accumulations ofnon-volatile metal such as copper, it can be permitted to accumulate upto the level of the slag tap hole, after which it will be vented alongwith the slag and form a regulus at the bottom of the slag pot.

The use of inert gas Ordinarily, diffusion of magnesium vapor alone issufi'icient to provide for the mass transfer from the reactor to thecondenser. However, if desired, a stream of inert gas may be introducedinto the furnace and fed through the condenser, in order to augment themagnesium flow to the condenser, in which case a recycle system may bedesired to recover inert gas.

In a further embodiment of the present invention, the product magnesiumis vaporized and evolves from the reaction medium through asubstantially static atmosphere of inert gas and passes predominately bydilTusion through the inert gas from the reaction zone to a condensationzone and is collected. This modification allows operation at a higherabsolute pressure and also yields a magnesium product relatively pure ascompared with commercial magnesium, containing impurities, notablysilicon, in low concentration; see my co-pending application Ser. No.26,118, filed on Apr. 6, 1970.

Other embodiments using inert gas are also possible, for example, toincrease the partial pressure of the system, as disclosed in myco-pending application Ser. No. 796,214, US. Pat. No. 3,658,509, or asanother method of maintaining a positive pressure in the system duringslag tapping. In this use, the excess gas would be added to the systemjust before the slag is to be tapped, and removed, for instance bybleeding or by a. vacuum pump, before charging of the reactants isrecommenced.

Examples The following operations are conducted in an electric reducingfurnace coupled with a condensing chamber. The procedure is to chargethe furnace with components to produce a slag of the desiredcomposition, and to supply heat until a proper viscosity is reached at atemperature above about 1300" C., whereupon the oxide charge and thereducing agent are charged continuously in intermittent, small batches.When a large quantity of slag accumulates, the reaction is allowed torun until all or substantially all of the magnesium oxide content of theslag is reduced to magnesium. The major portion of the molten slag, andof spent alloy, if any, is then tapped, leaving enough slag and alloy torepeat the cycle. The addition of the oxide charge and the reductant,and the tapping of the slag and alloy are conducted in such a mannerthat, disregarding the fluctuations in the level of magnesium oxide, thecomposition of the slag is maintained substantially constant. Theoperation is conducted substantially continuously.

Attached Table II summarized stoichiometric data of examples having rawmaterials and slag compositions chosen to illustrate limiting and normalslag compositions; the use of pure aluminum 99%) and of three alloyswith extreme compositions with respect to alloying ingredients; and theuse of a low-grade magnesia produced by calcining magnesite containingabout 8 percent of S10 It has been assumed that all of the CaO requiredfor the the Si is supplied by using a low-grade siliceous magnesia whichat certain locations could provide a substantial cost advantage. Themaguesite used to produce the calcined magnesia could contain nearly 9percent S102. In Example E the SiO is supplied by using as a supplementary reductant the silicon contained in a silicon-aluminum alloy,which can also result in a substantial cost advantage.

For a further example, as a modification of Example D, a calcinedmagnesia containing about 27 percent SiO could be used to produce slaghaving the composition: A1 0 49 percent; CaO, 41 percent; SiO percent.This requires additional dolime to provide CaO to bind TABLEIL-STOICHIOMETRIC DATA FOR REACTIONS USING TYPICAL ALLOYS, DOLIME ANDMAGNESIA Example A B C D E F G Alloy number 1100 1100 1100 1100 13 2207075) Al, percent 100 100 100 100 88 90 9 Mg, per 10 2. 5 Si, percent 12Zn, percent... 5. 6 C11, percent 1. 6 Other 0. 3C! lmo, 5o 57. 5 e5 5555 57. 5 60 CaO---.- 50 42. 5 37.9 36.5 42.5 SiOz 7. 1 8. M.p., C 1, 400-1, 530 -1, 600 -1, 550 -1,550 -l, 500 -1, 550

Units per magnesium produced Raw materials:

Dolime 2. 1. 80 1. 33 1. 69 1. 38 1. 65 1. 48 Magnesia 0. 64 0. 92 1. 110. 97 1. 10 0. 85 0. 96 Alloy 0. 75 0. 75 0. 75 0. 75 0. 73 0. 77 0. 82Products:

Slag 2. 84 2. 47 2. 19 2. 58 2. 21 2. 27 2. 315 Magne 1 00 1. 00 1. 00l. 00 1. 00 1. 00 1 1. 045

sium 1 Case G-1.00 units magnesium and 0.045 units zine.

A and C. Case D slag derives S102 CaO'AhOa (64.6% A1203). Case B sla isthe median composition between rom 1.14 units magnesia containing 16%S102. Case E assumes 100% utilization of Si content of alloy. Cases Dand E slag will disintegrate.

slag will be derived from calcined dolomite, i.e. dolime, and that anyadditional MgO required will be supplied as comparatively pure MgOderived from sea water, bitterns, or other sources. These examples aregiven to illustrate the flexibility of the process with respect to rawmaterials on the one hand, and on the other hand to show the narrowrange of slag compositions from which nearly complete extraction of A1 0can be obtained.

Example A shows a slag which will solidify as crystalline 12 CaO.7Al Owhich is completely soluble in soda ash solutions. It represents thelower desirable limit of A1 0 for a silica-free slag, because if CaO isgreater than about percent the relatively insoluble compound 2CaO.AlOwill be formed, thereby decreasing A1 0 recovery.

Example C shows a slag which will solidify as crystalline CaO.Al O whichis also completely soluble in soda ash solution. It represents the upperdesirable limit of A1 0 for a silica-free slag, because if CaO is lessthan about 35 percent, the relatively insoluble compound CaO.2Al O willbe formed, thereby decreasing A1 0 recovery.

Example B shows a median slag between A and C, where the solid slag willbe a mixture of crystalline 12CaO.7Al O and CaO.Al O both of which arecompletely soluble. The preferred slag of the invention is this type ofmixed composition, but it is of course possible to operate somewhatoutside the indicated preferred limits without departing from the spiritof the invention.

Examples D and E show slags containing enough SiO to causedisintegration, thus avoiding the necessity of grinding anon-disintegrating slag. But in such slags the A1 0 content is notcompletely recoverable, and there is an additional sludge problem.However, in Example D the extra SiO and about 1.46 units of siliceousmagnesia after adjusting for the MgO content of the additional dolime.The slag ratio will thus be roughly 2.9 per unit of magnesium but alow-grade magnesite containing as much as 15 percent SiO could be usedto provide the siliceous magnesia.

If A1 0 is extracted from the lag, and the residual CaCO is calcined andrecycled, there is substantially no residue. The process is then aclosed cycle, with MgO and Al in, Mg and A1 0 out.

Moreover, as mentioned previously, the Al O byproduct can be reduced toprovide an aluminum recycle to reducing agent feed.

Data for Examples 1-7 are shown in Table III. In the reducing agent, thematerials other than aluminum and silicon metal are omitted; and in theoxide charge, it is assumed that the magnesia and dolomitic lime arepure MgO and MgO'CaO, respectively, in that the impure oxides areomitted. The composition of the slag resulting from complete reaction ofthe reducing agent with the MgO in the oxide charge is given. In orderto obtain the proper calcium-aluminate slag for Example 3 an additionalquantity of lime was added.

It can be seen that a very high recovery of alumina is obtained. It canalso be seen how the removal of recoverable alumina from the slagreduces the total slag ratio, as represented by the waste slag produced,to a very low level, in some cases approaching or even exceeding onepart waste slag by weight per part of magnesium produced.

In Example 4, the insoluble 2CaO-Si0 constitutes about 18 percent of theslag by weight. Since this increases the resulting waste slag ratio, itis apparent why the substantially pure aluminum is preferred overreducing agents containing relatively high amounts of silicon metal forthe present process.

The figures The attached figures illustrate schematically analuminothermic process, broken into two parts, embodying the presentprocess. It can be seen that the products of the main part aremagnesium, calcium aluminate and a small amount of residual metal (FIG.1). The calcium aluminate may be treated (FIG. 2) to produce cell-gradealumina and CaCO or CaO-the latter of which may be recycled to supplypart of the lime of the first part (FIG. 1). Thus, the net effect,somewhat simplified, can be to produce magnesium and cell-grade aluminafrom aluminum and magnesiathe products may be pure, but the reactantsneed not be.

TABLE III Example 1 2 a 4 5 e 7 Reducing agent $1.--. 0.3 15.6 15.0 1013.8 5.4 11.7 Oxide charge:

Dolomite lime 2.58 0 1.88 1.95 1.50 1.42 CaO added 0 0 1.31 0 0 0 0 s10,added 0 0 0 0 0 0 0 810. 3.2 8.4 8.6 6.2 3.5 3.7 7.4 Slag produced--.3.06 2.87 2.69 2.52 2.52 2.30 2. 24 Percent A120:

recoverablem- 62.7 7.45 89.8 90 91.0 97.5 44.2

Wasteslagproduced.-- 2. 2.04 1.67 1.39 1.45 0.99 1.68

l Expressed as unit weight per unit of magnesium produced. 3 The slagpercent is calculated as follows: SiOr+Ah0z+Ca0-(Ca0 equivalent for anyT102 present) =1007 I 15.0 grams of Slag leached with 11.6 grams No.00.in 250 milliliters or 11.0 at 25 0&5 o. for 16 hours.

4 Waste slag produced is calculated as follows: Slag produced-(WeightA1101) X(percent A110; recoverable).

I Examples 2 and 3 show the difiiculty presented when a slag is notproperly crystallized to improve the recovery of aluminum oxide. Slag No. 2 was not cooled slowly enough to permit adequate crystallization.whereas No. 3 was cooled slowly under proper control.

I The silicon dioxide content of these slags is too low to produceappreci' able dusting, even when the slag is cooled slowly.

I claim:

1. An aluminothermic process for the production of magnesium, whichcomprises charging to a reactor an oxide mixture containing magnesiumoxide and calcium oxide, and a metallic reducing agent comprising atleast 85 percent aluminum; maintaining in said reactor a moltencalcium-aluminate slag bath; removing, when the mag nesium oxide contentof said slag is less than 5 percent, calcium aluminate comprising 35-65percent A1 0 35- 55 percent CaO and 0-10 percent SiO- evolving magnesiumvapor from said reactor to a condenser; and condensing and recoveringsaid magnesium.

2. The process of claim 1, wherein said oxide mixture comprises MgO andCaO in a ratio by weight of between 1.121 and 23:1.

3. The process of claim 1, wherein said removed calcium aluminatecomprises 51-65 percent A1 0 and 35- 49 percent CaO.

4. The process of claim 1, wherein said reactor and condenser containinert gas at a partial pressure of at least 0.1 atm.

5. The process of claim 1, wherein evolution of said magnesium vapor isinduced by a stream of inert gas passing from said reactor to saidcondenser.

6. The process of claim 1, wherein said magnesium vapor evolves fromsaid reactor to said condenser predominately by diifusion through asubstantially static atmosphere of inert gas.

7. The process of claim 1, wherein said oxide mixture comprises magnesiaand calcined dolomite.

8. The process of claim 1, wherein said magnesium oxide is derived atleast in part from a siliceous magnesite.

9. The process of claim 1, wherein said oxide mixture comprises freelime.

10. A process for the production of magnesium and alumina, whichcomprises:

(a) charging to a reactor an oxide mixture containing magnesium oxideand calcium oxide, and a metallic reducing agent comprising at least 85percent aluminum; maintaining in said reactor a molten calcium-aluminatesla-g bath; removing, when the magnesium oxide content of said slag isless than 5 percent, calcium aluminate comprising 35-65 percent Al O35-55 percent CaO and 01() percent SiO evolving magnesium vapor fromsaid reactor to a condenser; and condensing and recovering saidmagnesium; and

(b) solidifying and grinding or dusting said calcium aluminate intoparticulate form; leaching said particulate calcium aluminate to obtainAl(OH) and calcining said Al(0I-I) 3 to obtain purified alumina.

11. The process of claim 10, wherein said removed calcium aluminatecomprises 51-65 percent A1 0 and 35-49 percent CaO.

12. The process of claim '10, wherein said reactor and condenser containinert gas at a partial pressure of at least 0.1 atm.

13. The process of claim 10, wherein calcium oxide is recovered fromsaid particulate calcium aluminate and recycled to constitute a portionof said oxide mixture.

14. The process of claim 10, wherein said purified alumina is reduced toaluminum metal, and said aluminum metal is recycled to constitute aportion of said metallic reducing agent.

References Cited UNITED STATES PATENTS 3,441,402 4/1969 Magee et a1. -673,579,326 5/1971 Avery 75-67 L. DEWAYNE .RUTLEDGE, Primary Examiner M.I. ANDREWS, Assistant Examiner

